An organic electroluminescence device (hereinafter, “electroluminescence” may be abbreviated as “EL”) is a spontaneous light emitting device which utilizes the principle that a fluorescent substance or a phosphorescent substance emits light by energy of recombination of holes injected from an anode and electrons injected from a cathode when an electric field is applied. Since an organic EL device of the laminate type driven under a low electric voltage was reported, many studies have been conducted on organic EL devices using organic materials as the constituent materials. The devices of the laminate type use tris(8-quinolinolato)aluminum for a light emitting layer and a tetraphenyldiamine derivative for a hole transporting layer. Advantages of the laminate structure are, for example, that the efficiency of hole injection into the light emitting layer can be increased, that the efficiency of forming excitons which are formed in the light emitting layer by blocking and recombining electrons injected from the cathode can be increased, and that excitons formed in the light emitting layer can be confined. As described above, for the structure of the organic EL device, a two-layered structure having a hole transporting (injecting) layer and an electron transporting light emitting layer and a three-layered structure having a hole transporting (injecting) layer, a light emitting layer, and an electron transporting (injecting) layer are well known. To increase the efficiency of recombination of injected holes and electrons in the devices of the laminate type, the structure of the device and the process for forming the device have been studied.
As a light emitting material for an organic EL device, there are known light emitting materials including a chelate complex such as a tris(8-quinolinolato)aluminum complex, a coumarin derivative, a tetraphenylbutadiene derivative, a distyrylarylene derivative, and an oxadiazole derivative. It is reported that light emission ranging from blue light to red light in a visible light region can be obtained by using those light emitting materials, and a device exhibiting color images was realized.
A fluorescent light emitting material that emits light by means of a singlet exciton has been conventionally used as a light emitting material for an organic EL device. In recent years, the utilization of a phosphorescent light emitting material that emits light by means of a triplet exciton as well as the fluorescent light emitting material has also been proposed (for example, Non Patent Literatures 1 and 2). An organic EL device using the phosphorescent light emitting material can achieve luminous efficiency three to four times as high as that of an organic EL device using only the fluorescent light emitting material because it is assumed that singlet excitons and triplet excitons are produced at a ratio of 1:3 upon recombination of electrons and holes in an organic EL device by virtue of a difference in spin multiplicity. In blue phosphorescent light emission, however, high efficiency and a long lifetime are hard to achieve, and hence the development of a host material that achieves the high efficiency and the long lifetime has been desired.
Although Patent Literatures 1 and 2 each describe a compound having a carbazole skeleton substituted with carbazole at each of its 3- and 6-positions through an N atom (3,6-dicarbazolylcarbazole skeleton), none of the documents describes a compound in which the dicarbazolylcarbazole skeleton is bonded to dibenzofuran or dibenzothiophene on its N atom through a single bond or a linking group. Patent Literature 3 does not describe the dicarbazolylcarbazole skeleton. In addition, a light emitting device using any one of the compounds described in Patent Literatures 1 to 3 has been insufficient in efficiency and lifetime of blue phosphorescent light emission.